Applets

http://lectureonline.cl.msu.edu/~mmp/kap25/Snell/app.htm

http://www.physics.uoguelph.ca/applets/Intro_physics/refraction/LightRefract.html

http://www.physics.metu.edu.tr/~bucurgat/ntnujava/Lens/lens_e.html
For lenses and mirrors, in general - probably more complicated than you need, but this is what is going on in lenses (and mirrors).